A conventional optical unit has been commonly known in which, after light from a lamp passes through a first and a second lens array, a polarizing beam splitter (PBS), and a collimator lens, it is separated into R, B, and G light rays by means of plural dichroic mirrors; then the R, B, G light paths are altered through rotary prisms respectively so that the respective color light rays are directed onto a light valve device (hereinafter simply called “panel”) in different areas and the areas on which color light rays are thrown scroll in a prescribed direction on the panel sequentially.
The above conventional optical unit has an advantage that it uses a single-plate panel and is easy to assemble. However, since it requires plural rotary prisms, it cannot be compact. In addition, it uses not only plural rotary prisms but also many lenses and dichroic mirrors, so it is expensive and its light utilization efficiency is low due to the use of many lenses. Besides, the rotational positions of plural rotary prisms must be controlled in order to adjust the spots on the panel on which the R, G and B light rays are thrown and this adjustment is troublesome. Further, since plural motors are used, anti-noise measures must be taken.
Furthermore, in the conventional optical unit, R, G, and B light rays from dichroic mirrors overlap each other on a rotary polyhedron, causing mixing of the colors or contrast deterioration due to stray light.